The Events Calendar (v2 views): Add Thickbox to single event's featured image.

functions.php

<?php

/**
 * The Events Calendar (v2 views): Add Thickbox to single event's featured image (also make smaller and centered).
 *
 * @link  https://gist.github.com/cliffordp/b3745850fd2ab2aab1d4f104ed05ef93 This snippet.
 * @link  https://share.getcloudapp.com/E0uqlBJp Quick demo video.
 *
 * @see   cliff_tec_featured_image_thickbox_class()
 */
function cliff_tec_featured_image_thickbox() {
	// Protect against fatal if not on TEC v5.0+
	if ( ! class_exists( '\Tribe\Events\Views\V2\Template_Bootstrap' ) ) {
		return;
	}

	// If on single event page
	/** @var \Tribe\Events\Views\V2\Template_Bootstrap $template */
	$template = tribe( \Tribe\Events\Views\V2\Template_Bootstrap::class );

	if ( ! $template->is_single_event() ) {
		return;
	}

	// If event has a featured image
	if ( empty( tribe_event_featured_image() ) ) {
		return;
	}

	// Enqueue thickbox
	wp_enqueue_script( 'thickbox' );
	wp_enqueue_style( 'thickbox' );

	// Wrap featured image with full size image in thickbox
	add_action( 'wp_footer', 'cliff_tec_featured_image_thickbox_class' );
}

add_action( 'wp_enqueue_scripts', 'cliff_tec_featured_image_thickbox' );

/**
 * Convert the featured image to a clickable thickbox and add styling.
 *
 * If you don't want to add the styling, you may want to change the single event's featured image size in this file:
 * /wp-content/plugins/the-events-calendar/src/views/blocks/featured-image.php
 *
 * @see cliff_tec_featured_image_thickbox() Runs before this function.
 */
function cliff_tec_featured_image_thickbox_class() {
	$img_url = tribe_event_featured_image( null, 'full', false, false );

	$img_wrap = sprintf( '<a class="thickbox" href="%s"></a>', $img_url );

	// Thickbox already enqueues jQuery.
	// We make the Featured Image smaller and centered (but keep loading the 'full' size since another size, like 'medium' would be an additional HTTP call).
	?>
	<script>
		jQuery( document ).ready( function() {
			let tec_featured_img_wrap = jQuery( '.tribe-events-event-image' );

			tec_featured_img_wrap
				.find( 'img' )
				.wrap( '<?php echo $img_wrap; ?>' )
			;

			tec_featured_img_wrap
				.css( 'max-width', '400px' )
				.css( 'margin-left', 'auto' )
				.css( 'margin-right', 'auto' )
			;
		} );
	</script>
	<?php
}